Despite recent advances in the identification and refinement of nucleic acid therapeutics, finding suitable delivery means for these molecules in various applications has proved challenging. Moreover, while it is desirable to minimize the dosage of these expensive molecules, by localizing or targeting nucleic acid therapies to tissues/cells of interest, many technologies have been investigated, with few promising results.
RNAi [Fire A., et al (1998) Nature 391:801-11] has emerged as a means for sequence specific, posT transcriptional gene silencing, mediated by short interfering RNAs (siRNAs) homologous to the gene targeted for silencing. However, to be effectively used as drugs, the siRNAs (or their larger RNA precursors) must be delivered directly into the target cell. Targeted delivery of siRNA into specific cells of interest has been the main obstacle to achieving in vivo gene silencing by RNAi technologies. Specific delivery, dosage reduction, and minimizing toxicity are all important unmet objectives in this field.
Potential targeted siRNA delivery systems have emerged, such as antibody-mediated delivery, and liposomal delivery. Antibody mediated siRNA delivery may allow preferential accumulation of siRNA in target cells with less effect on normal tissues, and it has been suggested that such ligands can further be conjugated to delivery agents, such as liposomes, to promote uptake into target cells by receptor mediated endocytosis.
A further potential method for in vivo delivery of siRNA to specific target cells employs the nucleic acid binding properties of protamine, combined with the specificity of antibody-mediated delivery. Injection of siRNAs complexed with an antibody fragmenT protamine fusion protein have been used to selectively deliver siRNAs into target cells expressing the cell surface receptor recognized by the antibody [reviewed in Dykxhoorn, D. M., et al (2006) Gene Therapy 13-541-552; Song E. et al (2005) Nature Biotech. 23(6):709-717].
The specific cell type or targeted organ will generally vary with the type of therapeutic being delivered. For example, dendritic cells may be a key focus in cancer immunotherapy applications, as these potent antigen presenting cells are uniquely capable of inducing immunity to break tolerance to cancer antigens. It has been suggested that RNAi can be used for immune modulation by targeting gene expression in dendritic cells [Hill, J. A., et al (2003) J. Immunol. 171:691-696].
SOCS-1 has been shown to control the tolerogenic and immunogenic state of the dendritic cell, as well as the extent of antigen presentation and hence the magnitude of adaptive immunity [reviewed in Yoshimura, A., et al (2007) Nature Rev. Immunol. 7:454-465]. Silencing of SOCS-1 by siRNA enhances both antigen presentation by dendritic cells and antigen-specific anti-tumour immunity and may offer a selective means of breaking in host tolerance, of enhancing antigen-specific anti-tumour and anti-viral immunity, and of increasing the efficiency of dendritic cell-based cancer vaccines. Silencing SOCS-1 in dendritic cells may reduce the threshold of the cell's responsiveness to endogenous stimuli, permit persistent activation of antigen-specific T cells in vivo, and boost the anti-cancer activity of T cells.
In an ex vivo study, dendritic cells showed enhanced antigen-specific anti-tumour immunity when SOCS-1 was silenced in the dendritic cells before their vaccination with a cancer antigen [Shen, T. (2004) Nature Biotech 22(12): 1546-1553]. In an in vivo study in mice, silencing of SOCS-1 induced an anti-HIV-1 CD8+ and CD4+ T cell response as well as antibody responses [Song, X-T. et al (2006) PLoS Med 3:1-18].
The use of siRNA in the treatment of viral disease has also been suggested. In particular, the manifestation of chronic viral diseases relies on avoidance of the host immune system. It has been speculated that viral gene expression may be silenced by administration of virus-specific siRNA to the infected host.
In subjects with chronic viral or parasitic infections (where the organism is resident inside a host cell at some point during its life cycle), antigens are produced by and expressed in the host cell, and secreted antigens are present in the circulation. As an example, in the case of a chronic human hepatitis B virus (HBV) infected carrier, virions, HBV surface antigens, and a surrogate of the core antigens (in the form of the e-antigen) can be detected in the blood but are apparently tolerated by the host immune system.
Similarly, in cancer, tumour escape from immune surveillance and attack is a major determinant for tumour survival in the host. A need exists for new, therapeutically effective compounds, compositions and methods for eliciting or enhancing immune responses against infectious diseases or cancer, or to break tolerance to infectious diseases or cancer.